1. (Field of the Invention)
The present invention relates to a dielectric ceramic composition. More specifically, the invention relates to a dielectric ceramic composition which contains BaTiO.sub.3 as a chief component, as well as Nb.sub.2 O.sub.5 and/or Ta.sub.2 O.sub.5, MgO, oxides of rare earth elements and MnO, and which can be preferably used as a material of ceramic capacitors and resonators.
2. (Description of the Related Art)
Dielectric ceramic compositions have heretofore been used as materials for multi-layer ceramic capacitors. Such a multi-layer ceramic capacitor is fabricated by stacking plural pieces of a green sheet of a dielectric ceramic composition having an internal electrode formed therein such that a predetermined capacitance is obtained followed by firing as a unitary structure. For instance, a dielectric ceramic composition used for a multi-layer ceramic capacitor of X7R (EIA Standards: change of capacitance is within .+-.15% over a temperature range of from -55.degree. C. to +125.degree. C.) has a dielectric constant of as high as not smaller than 2500 at +25.degree. C., and has a thickness of not larger than 15 .mu.m per a piece of green sheet. It is important here that the firing temperature is not higher than, for example, 1300.degree. C.
That is, by employing a green sheet having a dielectric constant of not smaller than 2500 at 25.degree. C. and a thickness of not larger than 15 .mu.m, it is allowed to minimize the thickness of the raw green between the internal electrodes and to minimize the opposing areas and, hence, to realize a multi-layer ceramic capacitor in a small size. By selecting the firing temperature to be not higher than 1300.degree. C., furthermore, it is allowed to choose the internal electrodes from an increased variety of materials such as from an expensive material consisting of 100% of palladium through up to cheap Pd-Ag. In addition to the above, sufficient degree of attention must be given to properties of the dielectric ceramic composition such as dielectric loss tan .delta. and insulation resistance, and it is further desired that the dielectric loss is little dependent upon the AC voltage.
So far, dielectric ceramic compositions containing BaTiO.sub.3, Nb.sub.2 O.sub.5 and ZnO have been proposed already featuring improved dielectric constants (see Japanese Laid-Open Patent Publications Nos. 18162/1984, 18159/1984, etc.). Dielectric constants of from 2000 to 3000 can be obtained by using such dielectric ceramic compositions. Despite of their high dielectric constants, however, such dielectric ceramic compositions have large dielectric losses tan .delta. making it difficult to decrease the thickness of the green sheet. When used for the multi-layer capacitors, therefore, a large capacitance is not obtained.
In order to decrease the dielectric loss tan .delta., a dielectric ceramic composition containing BaTiO.sub.3, Nb.sub.2 O.sub.5, MgO and La.sub.2 O.sub.3 has been developed already (see Japanese Patent Publication No. 10766/1993). There has further been proposed a composition comprising BaTiO.sub.3 to which are added Nb.sub.2 O.sub.5 and MgO in such amounts that the molar ratio of Nb.sub.2 O.sub.5 /MgO is from 2.3 to 4 as well as rare earth oxides in amounts of from 0.1 to 0.5% by weight (see Japanese Patent Publication No. 19007/1980).
In fact, however, the above-mentioned dielectric ceramics all have dielectric constants of not larger than 2200, with which it is difficult to obtain a capacitor having a large capacitance in a small size. Besides, these dielectric ceramics still have such large dielectric losses that it is difficult to decrease the thickness of the green sheet.
Moreover, the above-mentioned dielectric ceramics all have AC voltage dependence of dielectric loss of not smaller than 3.0% under the application of a voltage of 2000 V/cm, making it difficult to decrease the thickness of the dielectric layer and, hence, making it difficult to obtain a capacitor having a large capacitance in a small size.